Heinlein made use of his engineering expertise to add realistic details to the story; for a time during World War II, he was a civilian aeronautics engineer working at a laboratory where pressure suits were being developed for use at high altitudes.

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Clifford "Kip" Russell, a bright high school senior with an eccentric father, enters an advertising jingle writing contest, hoping to win an all-expenses-paid trip to the Moon. He instead gets an obsolete, but genuine, used space suit. Though a few make fun of him, with the help of sympathetic townspeople, and using his own ingenuity and determination, Kip puts the suit (which he dubs "Oscar") back into working condition.

Kip reluctantly decides to return his space suit for a cash prize to help pay for college, but puts it on for one last walk. As he idly broadcasts on his shortwave radio, someone identifying herself as "Peewee" answers and requests a homing signal. He is shocked when a flying saucer lands practically on top of him. A young girl (Peewee) and an alien being (the "Mother Thing") flee from it, but all three are quickly captured and taken to the Moon.

Their alien kidnapper ("Wormface") is a horrible-looking, vaguely anthropomorphic creature who contemptuously refers to all others as "animals". Wormface has two human flunkies who assisted him in initially capturing the Mother Thing and Peewee, a preteen genius and the daughter of an eminent scientist. The Mother Thing speaks in what sounds to Kip like birdsong, with a few musical notations in the text giving the flavor of her language. Kip and Peewee have no trouble understanding her.

Kip, Peewee, and the Mother Thing try to escape to the nearest human base by hiking across the lunar surface, but they are recaptured and taken to a base on Pluto. Kip is thrown into a cell, later to be joined by the two human traitors, who have apparently outlived their usefulness. Before they later disappear, one mentions to Kip that his former employers eat humans.

The Mother Thing, meanwhile, makes herself useful to their captors by constructing advanced devices for them. In the process, she manages to steal enough parts to assemble a bomb and a transmitter. The bomb takes care of most of the Wormfaces, but the Mother Thing freezes solid when she tries to set up the transmitter outside without a spacesuit. Kip nearly freezes to death, too, while activating the distress beacon, but help arrives almost instantly. It turns out that the Mother Thing is far hardier than Kip had suspected, and was not in danger. Kip, however, suffers severe frostbite and is kept in a state of cryopreservation while the Mother Thing's people figure out how to heal him.

Front cover of the 1981 Del Rey edition

Kip and Peewee are transported to Vega 5, the Mother Thing's home planet. While Kip recuperates, "Prof Joe", a "professor thing", learns about Earth from Peewee and Kip. Once Kip is well, he, Peewee, and the Mother Thing travel to a planet in the Lesser Magellanic Cloud, to face an intergalactic tribunal, composed of many advanced species which have banded together for self-protection.

The Wormfaces are put on trial first. They promise to annihilate all other species, and are judged to be dangerous. Their planet is "rotated" out of three-dimensional space without their star - effectively an act of genocide dooming them to freeze to death.

Then it is humanity's turn, as represented by Peewee, Kip, Iunio (an Ancient Roman centurion), and a Neanderthal man. The Neanderthal is rejected as being of another species. Iunio proves belligerent, but brave. Peewee's and Kip's recorded remarks are then admitted into evidence. In humanity's defense, Kip makes a stirring speech. The Mother Thing and a representative of another race argue that the short-lived species are essentially children who should be granted more time to learn and grow. It is decided to re-evaluate humanity after "a dozen half-deaths of radium" (19200 years).

Kip and Peewee are returned to Earth with devices and equations provided by the Vegans. Kip passes the information along to Professor Reisfeld, Peewee's father. After listening to Kip and Peewee's story, Reisfeld arranges a full scholarship for Kip at the Massachusetts Institute of Technology, where Kip wants to study engineering and spacesuit design.

In 2010, it was announced that Star Trek writer Harry Kloor had written a script for a potential film adaptation and optioned the film rights. The film was expected to come out in 2013,[4][5] but as of the end of 2013 was still listed as "in development".